During the era when many nuclear reactor plants were constructed, techniques to decommission the nuclear reactors were generally not fully considered. Thus, many nuclear reactors were constructed without giving any consideration to dismantling the reactors at a future time.
One exemplary technique for decommissioning a nuclear reactor is via a “water platform” method, whereby the top of the reactor vessel is removed and the vessel is filled with water to provide a radioactive shield relative to the internal components of the reactor. A platform is placed on top of the water to provide access to reactor's internal components immediately underneath the platform. At this point, underwater cutting is performed on the portions of the reactor's internal components located immediately beneath the platform. The cut pieces are then loaded onto the platform and transferred to either a “wet cutting station” in which further underwater cutting is performed or a “dry cutting station” in which further cutting is performed in an air-controlled environment. The water level is then decreased, the platform lowered, and the cutting process is again initiated. This sequence of events is repeated until all of the reactor's internal components are removed. Thereafter, the cutting of the reactor vessel itself is initiated.
The water platform methodology is substantially limited in that it may result in human worker interaction with the sectioned reactor internal components, for example, during transfer of sectioned reactor components between the water platform and the wet/dry cutting stations. Additionally, the cutting process usually produces a significant amount of particles whereby respirators and HEPA ventilation are sometimes necessary to combat the effects of airborne contamination. Furthermore, the “shielding” water in the vessel will absorb particles produced during the cutting process whereby constant circulation and filtration of this fluid is necessary to remove liquid radioactive waste.
An alternate method of decommissioning a nuclear reactor was used on a retired nuclear reactor at the Shippingport Power Plant. The Shippingport nuclear reactor was an offspring of the Eisenhower presidency and thus its decommissioning was orchestrated by the United States Department of Energy. In decommissioning this unit, the reactor vessel was filled with concrete and then moved in one piece to a disposal site. The Shippingport reactor, which was rated at 72 megawatts, was significantly smaller than most of the commercial nuclear reactors in use or construction today. Nonetheless, the weight of the reactor when filled with concrete required the fabrication of special lifting equipment to lift the reactor from its underground housing. More particularly, the project required the erection of a gigantic frame, the construction of four huge hydraulic jacks, each having an approximately 6,000 ton lifting capacity, and the mounting of these jacks on the frame. In the transfer of the Shippingport reactor to the transport vehicle (a barge in this case), the jacks hoisted the reactor seventy-seven feet into the air, moved it approximately thirty-eight feet horizontally along a track and then lowered it onto a trailer.
The Energy Department's decision to decommission the Shippingport reactor in this manner, which avoided cutting apart the radioactive structure, saved an estimated seven million dollars and, perhaps more importantly, dramatically reduced worker exposure to radiation. However, such a procedure is probably not possible for most commercial reactors having an average rating of approximately 1000 megawatts, and would weigh over 2500 tons if filled with concrete. Moreover, even if larger reactors could be moved in one piece, the capital cost of fabricating the necessary lifting equipment would likely make such an approach economically unfeasible.